There are numerous devices for forming circles in the prior art. Some of these devices require sharp pivot points to which pivot arms are connected with marking means for inscribing the circles. However, these devices are unsuitable for use on hard surfaces, such as, for example, on the glass or mylar surfaces encountered in the use of overhead projectors. This is because the sharp pivot points deface the surface upon which the circle is being inscribed by leaving small holes in the Mylar surface or permanently scratching the glass surface. In addition, the sharp points of these devices render them unsafe.
Consequently, devices for forming circles without sharp points have been designed in an effort to circumvent the problems associated with such prior art devices. One such device is disclosed in U.S. Pat. No. 4,353,166 and comprises a flat member with a circular hole adjacent one end. A rotatable disk is releasably retained within the hole. A series of small holes sized to receive a marking instrument are spaced from the center point of the disk to the far end of the flat member at specific intervals. In use, the disk is held in the desired location and large circles are formed by holding the flat member in the desired location, inserting the marking instrument into the desired hole and rotating the flat member around the disk. Small circles are formed by inserting the marking instrument into the desired hole in the disk and rotating the disk within the flat member.
Another device for forming circles is disclosed in U.S. Pat. No. 4,530,156 which comprises a flat member with a first rotatable disk having a series of holes spaced at intervals and sized to receive a marking instrument. A second rotatable disk is disposed within the first rotatable disk and has at least one hole for receiving a marking instrument. The rotational position of the second disk may be changed relative to the first disk for adjusting the spacing of the hole in the second disk from a center point on the first disk.
While the above devices provide a means for forming circles on mylar, glass, and other delicate surfaces, there are problems associated with their use. For example, these devices frequently slide on the surface during circle formation. This sliding movement results in assymetrically formed circles. Moreover, since such surfaces often are non-absorbent (unlike e.g. paper) smearing of the still-wet lines on the drawing surface often occurs as the flat members and/or rotatable disks rub against the drawing surface during rotation. Such smearing is especially prevalent with the wet ink of felt or nylon tipped pens used on drawing surfaces displayed using overhead projectors.
Accordingly, an object of the present invention is to provide a device for forming circles that firmly engages the drawing surface to stabilize the device over the surface during circle construction.
It is another object of the present invention to provide a device for forming circles that prevents smearing of the still-wet lines on the drawing surface during formation of the circles.
It is yet another object of the present invention to provide a device for forming circles that facilitates locating the device over the center point of a circle and/or marking the center point of a circle with a marking instrument.
It is a further object of the present invention to provide a device for forming circles that is relatively inexpensive.